Method of manufacturing particulate matter detection element

ABSTRACT

A method of manufacturing a particulate matter detection element for detecting particulate matter in a gas to be measured includes manufacturing flat-shaped conductor layers, flat-shaped insulating layers, a laminated structure in which the conductor layers and the insulating layers are alternately laminated, and a detecting unit having the conductor layers of different polarities as a pair of detection electrodes on a cross section of the laminated structure. The conductor layers each have a constant thickness, and include conductor layer planar portions having a stripped-pattern cross section, and tapered conductor layer end edge portions each having a triangular cross section, provided on both sides of the respective conductor layer planar portions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. application Ser. No.15/110,447, filed Jul. 8, 2016 which is the U.S. national phase ofInternational Application No. PCT/JP2015/050508, filed Jan. 9, 2015,which designated the U.S. and is based on and claims the benefit ofpriority from earlier Japanese Patent Application No. 2014-002882, filedJan. 10, 2014, the disclosures of each of which are incorporated hereinby reference.

BACKGROUND OF THE INVENTION Technical Field

The present disclosure relates to a particulate matter detection elementfavorably used for an exhaust gas purification system of a vehicleinternal combustion engine to detect particulate matter present in anexhaust gas that is a gas to be measured, and relates to a particulatematter detection sensor, and a method of manufacturing the particulatematter detection element.

Background Art

Particulate matter detection sensors that detect particulate matter invarious gases to be measured have been proposed. In such a particulatematter detection sensor, a pair of electrodes are formed on a surface ofa substrate having insulating properties. Taking advantage ofparticulate matter having electrical conductivity, the particulatematter detection sensor senses changes in electrical characteristics,such as resistance and capacitance, caused by particulate matter beingdeposited between the pair of electrodes to thereby detect theparticulate matter contained in a gas to be measured, such as acombustion exhaust gas of an internal combustion engine.

For example, a patent literature JP-A-2008-502892 discloses a sensorelement in which a pair of comb-shaped electrodes are formed on aninsulated substrate such as of alumina ceramic.

In the sensor element of the patent literature mentioned above, avoltage is applied across the pair of electrodes from a power supplyunit to form a non-uniform electric field in a space between thecomb-shaped electrodes meshing with each other. Thus, soot particlescontained in an exhaust gas passing through the sensor element areattracted to the electrodes and deposited thereon. Detecting theresistance across the electrodes of this moment, the amount of depositedsoot can be measured.

On the other hand, a patent literature JP-A-S60-196659 disclosesresistance measurement electrodes for use in a sensor. The resistancemeasurement electrodes have a laminated structure in which conductorlayers and insulating layers are alternately laminated using thick-filmprinting and green sheets to accurately form electrodes with a distanceof 50 μm or less therebetween, which has been difficult to achieve withconventional thick-film printing. A cross section of the laminatedstructure is used as the resistance measurement electrodes, with theconductor layers serving as the electrodes. The patent literatureJP-A-2008-502892 discloses that the distance between the electrodes canbe reduced to about 10 μm which is determined by the thickness of theinsulating layer.

CITATION LIST Patent Literature

[PTL 1] JP-A-2008-502892

[PTL 2] JP-A-S60-196659

A laminated structure can be formed by alternately laminating conductorlayers and insulating layers, with the conductor layers being exposed toa cross section of the laminated structure for use as a pair ofelectrodes as disclosed in JP-A-S60-196659. With this structure, avoltage can be applied across the electrodes to form an electric fieldto deposit particulate matter between the electrodes as disclosed inJP-A-S60-196659. However, in this case, electric charge is concentratedat corners of the electrode end portions.

It has been found that such electric charge concentration tends to causeparticulate matter to be locally deposited near the electrode endportions where electric field intensity is high. Thus, there is aconcern that the differences between the masses to which the detectionis sensitive and insensitive is increased and detection accuracy isdeteriorated.

Hence it is desired to provide a particulate matter detection elementhaving a laminated structure in which flat-shaped conductor layers andflat-shaped insulating layers are alternately laminated, the structurehaving a cross section where the conductor layers are exposed as a pairof electrodes to configure a detecting unit, with each electrode layerend portion being in a characteristic shape to minimize electric fieldconcentration thereon, to provide a particulate matter detection sensorthat uses the particulate matter detection element to form an electricfield by applying a high voltage across the pair of electrodes tocollect particulate matter, while detecting electrical characteristicschanging with the amount of particulate matter in a gas to be measureddeposited between the electrodes to highly accurately detect theparticulate matter, and to provide a method of manufacturing theparticulate matter detection element that minimizes concentration ofelectric charge on the electrode end portion to realize high detectionaccuracy.

A particulate matter detection element of the present disclosure has alaminated structure in which flat-shaped conductor layers andflat-shaped insulating layers are alternately laminated. Using a crosssection of the laminated structure, a detecting unit having theconductor layers of different polarities as a pair of detectionelectrodes is configured. Electrical characteristics changing with theamount of particulate matter deposited in the detecting unit aremeasured and for use in detecting particulate matter in a gas to bemeasured. The particulate matter detection element is characterized inthat the conductor layers each have a constant thickness, and includeconductor layer planar portions having a stripped-pattern cross section,and tapered conductor layer end edge portions each having a triangularcross section, provided on both sides of the respective conductor layerplanar portions.

In the present disclosure, the conductor layers may also each have aconstant thickness, and include conductor layer planar portions having astripped-pattern cross section, and gently curved conductor layer endedge portions each having a circular-arc cross section, provided on bothsides of the respective conductor layer planar portions.

Effects of the Invention

According to the present disclosure, electric field concentration isminimized in the conductor layer end edge portions, and variation ininsensible mass due to local deposition of particulate matter isminimized in electric field concentration portions. Therefore, aparticulate matter detection element having stable detection accuracycan be realized.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1A is a schematic diagram illustrating a general configuration of aparticulate matter detection sensor 1, according to a first embodimentof the present disclosure;

FIG. 1B is an enlarged perspective view illustrating a detecting unit 13that is a major part of a particulate matter detection element 10 usedin the particulate matter detection sensor 1 illustrated in FIG. 1A;

FIG. 1C is an exploded perspective view illustrating an example of aninner structure of the particulate matter detection element 10 used inthe particulate matter detection sensor 1 illustrated FIG. 1A;

FIG. 2A is an enlarged view illustrating a major part of a conventionalparticulate matter detection element 10 z in which an electrode layerend face is in a square shape, according to comparative example 1;

FIG. 2B is an enlarged view illustrating a major part of the particulatematter detection element 10 in which an electrode layer end face is inan obtuse-angle triangular shape, given as example 1 of the presentdisclosure;

FIG. 2C is an enlarged view illustrating a major part of a particulatematter detection element 10 a in which an electrode layer end face is inan acute triangular shape, according to example 2 of the presentdisclosure;

FIG. 2D is an enlarged view illustrating a major part of a particulatematter detection element 10 b in which an electrode layer end face is ina circular-arc shape, according to example 3 of the present disclosure;

FIG. 3A is an enlarged view illustrating a major part of a conventionalparticulate matter detection element 10 y in which an electrode layerend face is in a square shape and the end face position is not fixed,according to comparative example 2;

FIG. 3B is an enlarged view illustrating a major part of a particulatematter detection element 10 c in which an electrode layer end face is inan obtuse triangular shape and the end face position is not fixed,according to example 4 of the present disclosure;

FIG. 3C is an enlarged view illustrating a major part of a particulatematter detection element 10 d in which an electrode layer end face is inan acute triangular shape and the end face position is not fixed,according to example 5 of the present disclosure;

FIG. 3D is an enlarged view illustrating a major part of a particulatematter detection element 10 e in which an electrode layer end face is ina circular-arc shape and the end face position is not fixed, accordingto example 6 of the present disclosure;

FIG. 4A is a schematic diagram illustrating electric field intensitydistribution on a detecting unit plane, according to comparative example1;

FIG. 4B is a schematic diagram illustrating electric field intensitydistribution on a detecting unit plane, according to example 1;

FIG. 4C is a schematic diagram illustrating electric field intensitydistribution on a detecting unit plane, according to example 2;

FIG. 4D is a schematic diagram illustrating electric field intensitydistribution on a detecting unit plane, according to example 3;

FIG. 5A is a schematic diagram illustrating electric field intensitydistribution on a detecting unit plane, according to comparative example2,

FIG. 5B is a schematic diagram illustrating electric field intensitydistribution on a detecting unit plane, according to example 4;

FIG. 5C is a schematic diagram illustrating electric field intensitydistribution on a detecting unit plane, according to example 5;

FIG. 5D is a schematic diagram illustrating electric field intensitydistribution on a detecting unit plane, according to example 6;

FIG. 6 is an enlarged schematic perspective view illustrating adetecting unit 13 f, according to example 7;

FIG. 7A is a characteristics diagram illustrating variation in sensoroutput, according to comparative example 1 and example 1;

FIG. 7B is a characteristics diagram illustrating effects of reducingvariation of insensible mass, according to several comparative examplesand the present disclosure;

FIG. 8A is a schematic diagram illustrating a manufacturing process,according to comparative example 1;

FIG. 8B is a schematic diagram illustrating a manufacturing process,according to comparative example 3;

FIG. 8C is a schematic diagram illustrating a manufacturing process,according to example 1 of the present disclosure;

FIG. 8D is a schematic diagram illustrating a manufacturing process,according to example 2 of the present disclosure;

FIG. 9A is a schematic plan view illustrating a thick-film printingscreen used in manufacturing the particulate matter detection element ofthe present disclosure;

FIG. 9B is a set of diagrams including a cross-sectional view takenalong the line B-B of FIG. 9A, and cross-sectional and plan viewsillustrating an insulating layer with a conductor layer being formedcorresponding to the B-B cross-sectional view; and

FIG. 10 is a plan view illustrating a modification of the thick-filmprinting screen used in the present disclosure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to FIGS. 1A, 1B, and 1C, hereinafter is described anoutline of a particulate matter detection sensor 1 according to a firstembodiment of the present disclosure and a particulate matter detectionelement 10 that is a major part of the present disclosure.

The particulate matter detection sensor 1 (hereafter referred to assensor 1) of the present disclosure is configured by the particulatematter detection element 10 (hereafter referred to as element 10), apower supply 2, and a measuring unit 3. The element 10 includes adetecting unit 13 which is disposed in a gas to be measured that is anexhaust gas of an internal combustion engine. The power supply 2 appliesa predetermined voltage to the element 10. The measuring unit 3 measureselectrical characteristics, such as changes in current flowing throughthe element 10, and changes in voltage and impedance of the element 10,to detect particulate matter in the gas to be measured.

Electrical characteristics changing with the amount of particulatematter deposited in the detecting unit 13 of the element 10 are measuredby the measuring unit 3 to detect particulate matter in the gas to bemeasured.

In the description hereinafter, the element 10 side provided with thedetecting unit 13 and exposed to the gas to be measured is referred toas a tip end side. The element 10 side connected to the power supply 2and the measuring unit 3 is referred to as a base end side.

The sensor 1 can be arranged downstream of a diesel particulate filter(DPF) to detect abnormality of the DPF. Alternatively, the sensor 1 canbe arranged upstream of the DPF and used in a system that directlydetects particulate matter PM flowing into the DPF.

When the sensor 1 is actually arranged in a flow path of a gas to bemeasured, a known configuration commonly used as a particulate matterdetection sensor including a housing or a cover protecting the detectingunit 13, not shown, can be appropriately used to fix the element 10.

Referring to FIG. 1B, characteristics of the element 10 that is a majorpart of the present disclosure will be specifically described.

The element 10 has a laminated structure in which flat-shaped conductorlayers 11 and 12 and flat-shaped insulating layers 100 are alternatelylaminated.

The element 10 uses its cross section to configure the detecting unit 13where the conductor layers 11 and 12 having differing polarities form apair of detection electrodes.

As shown in FIG. 1B, the detecting unit 13 is configured such that thecross sections of the conductor layers 11 and 12 are alternated, withthe insulating layer 100 being interposed between each pair of theconductor layers 11 and 12.

According to the present embodiment, the conductor layers 11 and 12 arecharacterized in that they each have a constant thickness and includerespective conductor layer planar portions 110 and 120 (hereafterreferred to as planar portions 110 and 120) and respective conductorlayer end edge portions 111 and 121 (hereafter referred to as end edgeportions 111 and 121). The planar portions 110 and 120 have crosssections in a stripped pattern. The end edge portions 111 and 121, whichare each tapered and have a triangular cross section, are provided toboth sides of the respective conductor layer planar portions 110 and120.

Since the conductor layer end edge portions 111 and 121 having atriangular cross section are provided to both end edges of therespective conductor layers 11 and 12, the present disclosure can reduceelectric field concentration at the end edges of the conductor layers 11and 12. Thus, particulate matter is prevented from being locallydeposited on electric field concentration portions (i.e., portions whereelectric field is concentrated) and detection accuracy is improved andstabilized.

Known conductive materials can be used as appropriate for the conductorlayers 11 and 12. For example, conductive materials that can be usedinclude metal materials such as aluminum, gold, platinum, and tungsten,metal oxide materials such as ruthenium oxide, and any perovskite-typeconductive oxide material selected from LNF (LaNi_(0.6)Fe_(0.4)O₃), LSN(LaNi_(0.6)Fe_(0.4)O₃), LSM (La_(1-x)Sr_(x)MnO_(3-δ)), LSC(La_(1-x)Sr_(x)CoO_(3-δ)), LCC (La_(1-x)Ca_(x)CrO_(3-δ)), and LSCN(La_(0.85)Sr_(0.15)Cr_(1-x)Ni_(x)O_(3-δ)) (0.1≤X≤0.7).

Materials that can be used, as appropriate, for the insulating layer 100include insulating layer materials such as alumina, magnesia, titaniaand mullite, dielectric materials each being a mixture of ahigh-dielectric constant material, such as barium titanate, with aluminaor zirconia, and known ceramic materials such as partially stabilizedzirconia, represented by 8YSZ (ZrO₂)_(0.82)(Y₂O₃)_(0.08)).

The present embodiment shows an example in which the detecting unit 13is formed such that the cross sections of the pair of conductor layers11 and 12 are exposed parallel to a lateral face on the tip end side ofthe particulate matter detection element 10 in a rectangularparallelepiped shape. However, the detecting unit 13 may be providedsuch that the cross sections of the pair of conductor layers 11 and 12are exposed from a bottom surface on the tip end side of the element 10.

In FIGS. 1A to 1C, different hatchings are used to clarify that the pairof conductor layers 11 and 12 are alternately laminated and thepolarities are alternated. The different hatchings are not used fordiscriminating the materials of the conductor layers 11 and 12.

Referring to FIG. 1C, an inner structure of the element 10 will be morespecifically described.

The insulating layer 100 is formed into a flat shape by a knownmanufacturing method, such as doctor blading, with through holes beingpunched, as necessary, in predetermined positions to thereby formthrough hole electrodes 114 and 124.

The pair of conductor layers 11 and 12 are configured by the planarportions 110 and 120 which are provided with the end edge portions 111and 121 using a manufacturing method described hereafter, lead portions112 and 122 connected to the outside, terminal portions 113 and 123, andthrough hole electrodes 114, 124, and 125.

The through hole electrodes 114 and 124 electrically conduct the planarportions 110 and 120 having the same polarity.

The lead portions 112 and 122, the through hole electrodes 114 and 124,and the terminal portions 113 and 123 are formed by a manufacturingmethod, such as known thick-film printing.

A laminated structure is used for the element 10. Specifically, in thelaminated structure, several conductor layers 11 and 12 are laminated onrespective insulating layers 100 such that the conductor layers 11 and12 are alternated. The present embodiment includes a lowermostinsulating layer 100H which is provided with a heating element 140 thatgenerates heat by energization and a pair of lead 141 and terminal 142for electrically conducting the heating element 140, thereby configuringa heating unit 14.

For the heating element 140, a known heating resistor material such astungsten, molybdenum silicide, or ruthenium oxide is used. For the lead141 and the terminal 142, a known electrically conductive metal materialsuch as gold, platinum, or tungsten is used. A known method, such asthick-film printing, is used for forming these components.

The element 10 is integrally formed by baking.

In forming the detecting unit 13 of the present embodiment, the element10, after being laminated and baked, is appropriately cut such that across section thereof is exposed to a lateral side face thereof,followed by polishing.

Referring now to FIGS. 2A, 2B, 2C, 2D, 3A, 3B, 3C, and 3D, hereinafterare described comparative example 1, example 1, example 2, example 3,comparative example 2, example 4, example 5, and example 6 through thestudy of which the advantageous effects of the present disclosure havebeen confirmed.

A basic structure of both the comparative examples and the examples is alaminated structure similar to that of example 1 shown in FIG. 1C. Inthe description below, for clarity's sake, corresponding portions arerepresented by reference signs suffixed with z, y, x, and w forcomparative examples 1, 2, 3, and 4, and suffixed with a to g forexamples 2 to 8.

In an element 10 z shown in FIG. 2A as comparative example 1, insulatinglayers 100 z are laminated with respective conductor layers 110 z andconductor layers 120 z, such that the conductor layers 110 z arealternated with the conductor layers 120 z.

The conductor layers 110 z and 120 z of the comparative example 1 eachhave a rectangular cross section, with end faces being in a square shapeand aligned.

In the element 10 shown in FIG. 2B as example 1, the insulating layers100 are laminated with the respective conductor layers 110 and theconductor layers 120, such that the conductor layers 110 are alternatedwith the conductor layers 120.

The conductor layers 110 and 120 of example 1 have the tapered end edgeportions 111 and 121, respectively, which have a triangular (obtuseangle) cross section and are aligned.

An element 10 a shown in FIG. 2C as example 2 is different from example1 in that end edge portions 111 a and 121 a have an acute triangularcross section.

An element 10 b shown in FIG. 2D as example 3 is different from example1 in that end edge portions 111 b and 121 b are curved and have acircular-arc cross section. The element 10 b is characterized in thatthe conductor layers each have a constant thickness, and includerespective conductor layer planar portions 110 b and 120 b in a strippedpattern in the cross section of the element, and smoothly curvedconductor layer end edge portions 111 b and 121 b having a circular arccross section, provided on both sides of the respective conductor layerplanar portions.

In an element 10 y shown in FIG. 3A as comparative example 2, conductorlayers 110 y and 120 y have square end faces similarly to comparativeexample 1. However, the element 10 y is different from comparativeexample 1 in that the end faces are not aligned.

In an element 10 c shown in FIG. 3B as example 4, conductor layers 110 cand 120 c are provided with tapered end edge portions 111 c and 121 chaving a triangular (obtuse angle) cross section similarly to example 1.However, the element 10 c are different from example 1 in that the endedge portions 111 c and 121 c are not aligned.

In an element 10 d shown in FIG. 3C as example 5, conductor layers 110 dand 120 d are provided with tapered end edge portions 111 d and 121 dhaving a triangular (acute angle) cross section similarly to in example2. However, the element 10 d is different from example 2 in that the endedge portions 111 d and 121 d are not aligned.

In an element 10 e shown in FIG. 3D as example 6, curved end edgeportions 111 e and 121 e have a circular-arc cross section similarly toexample 3. However, the element 10 e is different from example 3 in thatthe end edge portions 111 e and 121 e are not aligned.

Referring to FIGS. 4A, 4B, 4C, 4D, 5A, 5B, 5C, and 5D, hereinafter aredescribed differences between comparative example 1, examples 1 to 3,comparative example 2 and examples 4 to 6 on the basis of simulationresults, for electric field distribution generated on a detecting unitplane when a given voltage is applied across each pair of conductorlayers.

As shown in FIG. 4A, in comparative example 1, it has been found thatstrong electric field concentration occurs at corners of the conductorlayers 11 z and 12 z, and the electric field intensity is relatively lowin the area between a pair of planar portions 110 z and 120 z in whichthe electric field intensity is uniform.

As shown in FIG. 4B, in example 1, it has been found that the electricfield concentration is dispersed into three areas in each of the endedge portions 111 and 121, the ratio of electric field concentrationbecomes relatively low, and accordingly, the electric field strength inthe area between the planar portions 110 and 120 in which the electricfield intensity is uniform becomes relatively high.

As shown in FIG. 4C, in example 2, it has been found that the electricfield concentration is further reduced, and accordingly, the electricfield strength in the area between the planar portions 110 a and 120 ain which the electric field intensity is uniform is maximized.

As shown in FIG. 4D, in example 3 as well, it has been found thatelectric field concentration is reduced, and the electric field strengthin the area between the planar portions 110 b and 120 b in which theelectric field intensity is uniform becomes relatively high.

In the case where the end faces are not aligned, as shown in FIG. 5A, incomparative example 2, it has been found that the electric fieldconcentration is more reduced than in comparative example 1, and theelectric field intensity in the area between the planar portions 110 yand 120 y in which the electric field intensity is uniform becomesrelatively higher than in comparative example 1.

On the other hand, in examples 4 and 5, it has been found that moreelectric field concentration is caused than in examples 1 and 2, and theelectric field intensity in the areas between the planar portions 110 cand 110 d, and between 120 c and 120 d in which the electric fieldintensity is uniform becomes relatively lower than in examples 1 and 2.

However, in example 6, it has been found that the electric fieldconcentration is more reduced than in example 3, and the electric fieldintensity in the area between the planar portions 110 e and 120 e inwhich the electric field intensity is uniform becomes relatively higherthan in example 3.

Referring to FIG. 6, an element 10 f of example 7 of the presentdisclosure will be described.

In the present example, a shielding layer 14 is provided to thedetecting unit 13 f to cover all the end edge portions 111 f and 121 fand part of the planar portions 110 f and 120 f, that is to say, tocover the areas where the electric field intensity is non-uniform. Theshielding layer 14 is made of a known insulating material, such as glassor alumina, or the same material as the insulating layer 100.

The configuration provided with the shielding layer 14 can also be usedin any of the foregoing examples 1 to 6.

Referring now to FIGS. 7A and 7B, the results of the tests conducted toconfirm the advantageous effects of the present disclosure will bedescribed. Let us assume the case where the detecting unit 13 of theelement 10 is located in a flow path of a gas to be measured and exposedto a gas, with a predetermined voltage being applied to the detectingunit 13 from the power supply 2, and with a known amount of particulatematter being permitted to flow. In this case, there is an insensiblemass Q₀ (dead period) for which the particulate matter cannot bedetected until a fixed amount or more of the particulate matter isdeposited in the detecting unit 13.

In addition to comparative examples 1 and 2, and examples 1 to 7, thefollowing examples and comparative examples were also prepared. For eachof the examples and comparative examples, several samples were preparedand a given amount of particulate matter was inputted to the samples tomeasure the insensible mass Q₀. The examples and comparative examplesadditionally prepared were: comparative example 3 obtained by formingconductor layers similar to those of comparative example 1 withoutforming intermediate layers; comparative example 4 obtained by providingthe shielding layer 14 mentioned above to comparative example 2; andexample 8 obtained by providing a shielding layer to example 6.

As shown in FIG. 7A, in comparative example 1, it has been found that anaverage value μ₂ of the insensible masses Q₀ is small and theparticulate matter is detectable at an early stage, but a variation σ₂between the samples is great.

On the other hand, in example 1, it has been found that the averagevalue μ₁ of the insensible masses Q₀ is greater than that of theinsensible masses in comparative example 1, but the variation σ₁ betweenthe samples is much smaller.

The reason for this is estimated to be that, in comparative example 1, ahigh concentration of the electric field occurs at the corners of theend faces of the conductor layers 11 z and 12 z, the particulate matteris attracted to the electric charge collected on the surfaces at thecorners and locally deposited, and the local deposition forms aconduction path at an early stage.

However, the electric field concentration at the corners greatly variesbetween the samples and is considered unstable.

Therefore, the samples have been evaluated using a variationcoefficient. Specifically, each sample is evaluated by calculating avariation coefficient CV (100√σ²/μ) (%). The evaluation results areshown in FIG. 7B.

As can be seen, the variation in comparative example 2 is smaller thanin comparative example 1 but, in all examples 1 to 7, the variationcoefficient can be made smaller than in comparative examples 1 to 4.Thus, it will be understood that the present disclosure has an effect ofimproving reliability as a sensor.

It is considered that concentration of the electric field and localdeposition of particulate matter in the end portions of the conductorlayers 11 and 12 are reduced by providing the end edge portions 111,121, 111 a, 121 a, 111 b, 121 b, 111 c, 121 c, 111 d, 121 d, 111 e, and121 e having a triangular or circular-arc cross section to both endfaces of the respective conductor layers 11 and 12.

In the end edge portions 111 a and 121 a having an acute triangularcross section, there is a large distance between the apexes at whichelectric field concentration tends to occur. This is considered to bethe reason why a long time is taken for forming the conduction pathbetween the pair of end edge portions 111 a and 121 a.

The following description sets forth methods of manufacturing theparticulate matter detection elements 10 z, 10 y, 10, and 10 a providedas the foregoing comparative examples 1 and 2, and examples 1 and 2. Inthe following description, FIGS. 8A, 8B, 8C and 8D are referred to.

Comparative example 1 shows a basic method of manufacturing theparticulate matter detection element 10 z in which a cross section ofthe alternate lamination of the conductor layers 11 z and 12 z and theinsulating layers 100 z is used as the detecting unit 13 z.

An insulating material, such as alumina, is mixed with a known binder,plasticizer, dispersant, solvent, and the like, and stirred to form aslurry. The slurry is formed into a sheet shape by a known manufacturingmethod, such as doctor blading, thereby obtaining an insulating sheet100 zGS.

In a punching step P0 z, not shown, the insulating sheet 100 zGS ispunched using a die or the like to form in advance, as required, analignment guide for printing, through holes for embedding through holeelectrodes 114 z and 124 z that connect conductor layers of the samepolarity, and the like, and the insulating sheet 100 zGS is punched outinto a predetermined outer shape.

In a printing step P1 z, a conductor paste is injected from a thick-filmprinting screen in which a predetermined conductor pattern is formed totransfer conductor layer printed films 11 zPRT and 12 zPRT to theinsulating layer sheets 100 zGS.

At this time, as shown being enlarged in the circle, due to the effectsof the rheological characteristics and surface tension of the paste, thefilm thickness near the center is reduced and the film thickness nearthe outer periphery is increased, although very slightly.

In comparative example 1, intermediate layer 101 z is formed bythick-film printing, using a paste containing the same materials as theinsulating material for forming the insulating layer 100 z, so as tocover portions except for the conductor layers. The intermediate layer101 z has the same thickness as that of the conductor layer printedfilms 11 zPRT and 12 zPRT.

In the subsequent laminating and pressure-bonding step P2 z, theinsulating layer sheets 100 zGS are laminated such that the polaritiesof the conductor layer printed films 11 zPRT and 12 zPRT are alternated,followed by pressure-bonding using a die or the like.

In a baking step P3 z, a laminated structure obtained in this way isintegrated by simultaneously baking the conductor layers 11 z and 12 zand the insulating layers 100 z.

Subsequently, the integrated object is cut and polished, for example, toexpose cross sections of the conductor layers 11 z and 12 z forming thedetecting unit 13, thereby completing the element 10 z.

In comparative example 1, due to the provision of the intermediate layer101 z, the conductor layer printed films 11 zPRT and 12 zPRT and theconductor layer sheet 110 z are hardly deformed during lamination andpressure bonding, and the mechanical strength of the element 10 z isgood. The conductor layers 11 z and 12 z retain the stripped-patterncross section.

However, it has been found that, when a voltage is applied across theconductor layers, concentration of the electric field at the corners isgreat, and as described above, variation in insensible mass is great.

The element 10 y shown as comparative example 2 is based on amanufacturing method similar to that of comparative example 1. In thelaminating and pressure-bonding step, the conductor layers 11 y and 12 yare laminated with the end faces thereof being misaligned as designed.

Referring to FIG. 8B, an outline of a manufacturing process for theelement 10 x as comparative example 3 and problems of comparativeexample 3 will be described.

In comparative example 3, as shown in a printing step P1 x, only theconductor layers 110 x and 120 x are printed on an insulating layersheet 100 xGS that has been punched out into a predetermined shape.Thus, without providing an intermediate layer, the manufacturing processproceeds to a laminating and pressure-bonding step P2 x.

Conductor layer printed films 11 xPRT and 12 xPRT are harder than theinsulating layer sheets 100 xGS. Therefore, in the laminating andpressure-bonding step P2 x, the insulating layer sheets 100 xGS areelastically deformed when they are laminated and pressure-bonded.

Resultantly, the insulating layer sheets 100 xGS are adhered to eachother, embedding the conductor layer printed films 11 xPRT and 12 xPRTtherebetween.

At this time, as shown being enlarged in the circle of FIG. 8Billustrating the laminating and pressure-bonding step P2 x, gaps eachhaving a triangular cross section are formed on both sides of theconductor layer printed films 11 xPRT and 12 xPRT.

Through the subsequent baking step P3 x, the laminated body is sinteredand the gaps are reduced as a result of the surface areas of the gapsbeing reduced. However, the gaps are not completely eliminated and someremains as voids. Thus, there is a concern that the gaps can triggerdelamination.

Further, the end faces of the conductor layers 11 x and 12 x afterbaking become polygonal or irregularly shaped. Thus, similarly tocomparative example 2, concentration of the electric field at thecorners is easily caused.

Referring to FIG. 8C, an outline of a manufacturing process of example 1of the present disclosure will be described.

According to the present embodiment, a process similar to that ofcomparative example 1, i.e., a punching step P0, is performed to punchan alignment guide and through holes, as required, in an insulatinglayer sheet 100GS and to punch the insulating layer sheet 100GS into apredetermined outer shape. In the punching step P0, simultaneously withpunching the insulating layer sheet 100GS, a recessed sheet 100PCD isformed. The recessed sheet 100PCD is provided with a recess 101 at theposition corresponding to the position where the conductor layer 110 or120 is formed by printing. The recess 101 is in conformity with thepredetermined shape of the conductor layer end edge portion 111 or 121.

Specifically, the punching die is provided with a protrusion for formingthe recess 101, and the surface of the insulating layer sheet 100GS ispressed against the die.

Thus, a tapered surface that is sloped at a desired angle can be formedin the portion in which the conductor layer end edge portion 111 or 121is formed.

As a result, if normal thick-film printing is performed in a printingstep P1, the conductor layer end edge portion 111 or 121 side to be incontact with the insulating layer sheet 100GS is sloped conforming tothe shape of the recess 101.

Further, since the recess 101 is also formed on the underside of therecessed sheet 100PCD, when the recessed sheets are laminated in thelaminating and pressure-bonding step P1, adhesion is improved betweenthe conductor layer printed films 110 and 120, forming no voids, unlikein comparative example 2.

Furthermore, if an intermediate layer as used in comparative example 1is not provided, the insulating layer sheets 100GS can be easily adheredto each other.

As a result, when the conductor layers 11 and 12 and the insulatinglayers 100 are integrated in a baking step P3, the element 10 hardlycausing delamination can be formed.

In addition, since the conductor layer end edge portions 111 and 121 canbe formed into a tapered shape having a triangular cross section, theelement 10 that can reduce concentration of the electric field can beeasily realized.

Referring to FIG. 8D, an outline of a manufacturing process of example 2of the present disclosure will be described.

A conductor printing step P1 a according to the present embodiment isdifferent from the foregoing embodiments in that partially changedopening-ratio printing screens PPM and PPMA are used when the conductorlayers 11 and 12 are printed on the insulating layer sheet 100(GS). Inthe partially changed opening-ratio printing screens PPM and PPMA, amesh opening ratio is partially changed such that the film thicknessresulting from the printing is reduced at predetermined positions.

Referring to FIGS. 9A, 9B, and 10, the partially changed opening-ratioprinting screens PPM and PPMA will be described.

According to the present embodiment, the partially changed opening-ratioprinting screen PPM used can reduce the amount of conductor pasteinjected from the portions where the opening ratio is designed to below. Thus, the thickness of the conductor layer formed can be reduced,enabling formation of the conductor layer end edge portions 111 a and121 a tapered outward with acutely-angled sloped surfaces and having atriangular cross section.

As described above, the conductor layers 11 a and 12 a having theconductor layer end edge portions 111 a and 121 a with a triangularcross section are formed in the insulating layer sheets 100GS. Furtherthe insulating layer sheets 100GS are laminated and pressure-bondedusing a die or the like. Thus, the insulating layers 100GS are broughtinto intimate contact with each other, embedding the conductor layers 11a and 12 a therebetween.

At this time, since the conductor layer end edge portions 111 a and 121a are tapered and has a triangular shape, gaps as in comparative example2 are not formed.

Further, the laminated structure prepared in this way is baked in abaking step P3 a. Thus, the element 10 a reducing concentration of theelectric field near the conductor end faces can be quite easily formed.

Referring to FIGS. 9A and 9B, hereinafter are described characteristicsof a partially reduced opening-ratio screen M used in manufacturing theparticulate matter detection element 10 of the present disclosure, andthe shape of the conductor layers 11 and 12 formed by using the screenM.

It should be noted that the drawings to be referred to show only apattern for forming the conductor layer planar portion 110 and theconductor layer end edge portion 111 in one of the pair of conductorlayers 11 and 12. The pattern for forming the conductor layer planarportion 120 and the conductor layer end edge portion 121 of the otherconductor layer corresponds to a left-and-right reverse of the patternof the firstly mentioned conductor layer. Therefore, the followingdescription is provided omitting the secondly mentioned conductor layerand using a combined reference sign 110/120 for the commonconfiguration.

The partially reduced opening-ratio screen M used in the presentembodiment is obtained by partially rolling and smoothing a thick-filmprinting screen generally used in thick-film printing and by reducing amesh thickness and mesh opening ratio.

A resist film R is formed into a predetermined printed pattern bycoating an emulsion onto a printing screen, followed by exposure with apattern conforming to the configuration of the conductor layer planarportions 110 and 120, and further followed by curing.

As shown in FIGS. 9A and 9B, in a mask M2, portions where the conductorlayer end edge portions 111 and 121 are printed have a large thicknessin the cross-sectional direction and a large line width in the planardirection. Therefore, the opening ratio of the portions where theconductor layer end edge portions 111 and 121 are printed, that is,opening ratios P₁₁₁ and P₁₂₁ for forming the end edge portions are lowerthan the opening ratio of a mask M1 used for portions where theconductor layer planar portions 110 and 120 are formed, that is, openingefficiency P₁₁₀ and P₁₂₀ of forming a planar portion.

Therefore, as shown in FIG. 9B, when the conductor layer planar portions110 and 120 are printed, the amount of paste injected from the mask M2is reduced, and the thickness of the conductor layer end edge portions111 and 121 becomes smaller than the thickness of the conductor layerplanar portions 110 and 120.

Referring to FIG. 10A, a modification MA of the partially reducedopening-ratio screen will be described.

The foregoing embodiment has shown, as an example, the partially reducedopening-ratio screen M in which part of the thick-film printing screenis pressed to reduce the opening ratio. However, as shown in FIG. 10, ina partially reduced opening-ratio screen MA, an opening ratioP₁₁₁A/P₁₂₁A for forming an end edge portion can be design to be lowerthan an opening ratio P₁₁₀A/P₁₂₀A for forming a planar portion, byincreasing a weave density of lateral and vertical threads of a mask M2Afor printing the conductor layer end edge portions 111 and 121, comparedto a mask M1A for printing the conductor layer planar portions 110 and120.

In the partially reduced opening-ratio screen MA of the presentembodiment, the resist R is formed into a predetermined conductorpattern on a screen mesh whose opening ratio is partially adjusted inadvance.

Thus, when the conductor layers 11 and 12 are printed, the amount ofconductor paste injected from the mask M2A is reduced in the conductorlayer end edge portions 111 and 121, thereby forming the conductor layerend edge portions 111 and 121 with a thickness smaller than theconductor layer planar portions 110 and 120. Further, by graduallyincreasing the weave density outward, the conductor layer end edgeportions 111 and 121 can be gradually thinned outward, thereby formingthe tapered end edge portions with a triangular cross section.

The foregoing embodiments have shown methods by which the conductorlayer end edge portions 111 and 121 can be formed into a desired shapein the punching step P0 and in the printing step P1. These methods maybe combined.

In the laminating and pressure-bonding step, the conductor layers 11 and12 have been laminated and pressure-bonded after being dried, as anexample. However, the conductor layers 11 and 12 may be laminated andpressure-bonded in an undried state.

In particular, when the recess 10 is provided in the insulating layersheet 100(GS) in the punching step P0, and when the conductor layers 11and 12 are laminated and pressure-bonded in an undried state, theconductor layers 11 and 12 are deformed in a fluid manner conforming tothe shape of the recess 10. Therefore, the conductor layer end edgeportions 111 and 121 can be formed into a desired shape.

The foregoing embodiments have shown the methods in which theintermediate layer 101 made of an insulating material as in comparativeexample 1 is not used in the laminating and pressure-bonding process P3.However, when the conductor layer end edge portions 111 and 121 isformed into a tapered shape with a triangular cross section or a gentlycurved shape with a circular-arc cross section in advance, theintermediate layer 101 may be printed using a paste made of aninsulating material, in the printing step.

Use of the intermediate layer 101 can mitigate the shear stress appliedto the insulating layer sheet 100 during lamination andpressure-bonding, or can improve mechanical strength of the element 10,or can minimize formation of cracks in the baking step.

In addition to the improvement in detection accuracy, an effect ofimproving durability of the element 10 can be expected.

REFERENCE SIGNS LIST

-   -   1 particulate matter detection sensor    -   10 particulate matter detection element    -   100 insulating layer    -   11, 12 conductor layer    -   110, 120 conductor layer planar portion    -   111, 121 conductor layer end edge portion    -   13 detecting unit    -   14 shielding layer    -   2 power supply unit    -   3 measuring unit    -   P0 punching step    -   P1 conductor layer printing step    -   P2 laminating and pressure-bonding step    -   P3 baking step    -   P₁₁₁, P₁₁₁A, P₁₂₁, P₁₂₁A opening ratio for forming an end edge        portion    -   P₁₁₀, P₁₁₀A, P₁₂₀, P₁₂₀A opening ratio for forming a planar        portion    -   M, MA partially reduced opening-ratio screen.

1. A method of manufacturing a particulate matter detection element formeasuring electrical characteristics changing with an amount ofdeposited particulate matter, and for detecting particulate matter in agas to be measured, the method comprises: a punching step of punching atleast an alignment guide and a through hole, as required, in aninsulating layer sheet made of an insulating material, and punching theinsulating layer sheet into a predetermined outer shape; a thick-filmprinting step of injecting a conductor paste made of an electricallyconductive material onto the insulating layer sheet, which has beenobtained in the punching step, from a thick-film printing screen inwhich a predetermined conductor layer pattern is formed to form aconductor layer printed film of a predetermined shape; a laminating andpressure-bonding step of laminating and pressure-bonding the insulatinglayer sheets provided with the conductor layers, which have beenobtained in the thick-film printing step; and a baking step of bakingand integrating a laminated structure obtained in the laminating andpressure-bonding step, wherein simultaneously with punching theinsulating layer sheet, the punching step forms a recessed sheetprovided with a recess, which is in conformity with a predeterminedshape of the conductor layer end edge portion, at a positioncorresponding to the position where the conductor layer is printed.
 2. Amethod of manufacturing a particulate matter detection element formeasuring electrical characteristics changing with an amount ofdeposited particulate matter, and for detecting particulate matter in agas to be measured, the method comprises: a punching step of punching atleast an alignment guide and a through hole, as required, in aninsulating layer sheet made of an insulating material, and punching theinsulating layer sheet into a predetermined outer shape; a thick-filmprinting step of injecting a conductor paste made of an electricallyconductive material onto the insulating layer sheet, which has beenobtained in the punching step, from a thick-film printing screen inwhich a predetermined conductor layer pattern is formed to form aconductor layer printed film of a predetermined shape; a laminating andpressure-bonding step of laminating and pressure-bonding the insulatinglayer sheets provided with the conductor layers, which have beenobtained in the thick-film printing step; and a baking step of bakingand integrating a laminated structure obtained in the laminating andpressure-bonding step, wherein the thick-film printing step uses apartially reduced opening-ratio screen in which an opening ratio at aposition of forming the conductor layer end edge portion in thethick-film printing screen, that is, an opening ratio for forming an endedge portion is permitted to be lower than an opening ratio at aposition of forming the conductor layer planar portion, that is, anopening ratio for forming a planar portion, in conformity with apredetermined shape of a conductor layer end edge portion.
 3. A methodof manufacturing a particulate matter detection element for measuringelectrical characteristics changing with an amount of depositedparticulate matter, and for detecting particulate matter in a gas to bemeasured, the method comprises: a punching step of punching at least analignment guide and a through hole, as required, in an insulating layersheet made of an insulating material, and punching the insulating layersheet into a predetermined outer shape; a thick-film printing stepinjecting a conductor paste made of an electrically conductive materialonto the insulating layer sheet, which has been obtained in the punchingstep, from a thick-film printing screen in which a predeterminedconductor layer pattern is formed to form a conductor layer printed filmof a predetermined shape; a laminating and pressure-bonding step oflaminating and pressure-bonding the insulating layer sheets, on whichthe conductor layers are formed and obtained in the thick-film printingstep; and a baking step baking and integrating a laminated structureobtained in the laminating and pressure-bonding step, whereinsimultaneously with punching the insulating layer sheet, the punchingstep forms a recessed sheet provided with a recess, which is inconformity with a predetermined shape of the conductor layer end edgeportion, at a position corresponding to the position where the conductorlayer is printed, and the thick-film printing step uses a partiallyreduced opening-ratio screen in which an opening ratio at a position offorming the conductor layer end edge portion in the thick-film printingscreen, that is, an opening ratio for forming an end edge portion ispermitted to be lower than an opening ratio at a position of forming theconductor layer planar portion, that is, an opening ratio for forming aplanar portion, in conformity with a predetermined shape of a conductorlayer end edge portion.
 4. The method of manufacturing a particulatematter detection element according to claim 1, wherein, in thelaminating and pressure-bonding step, the conductor layers are laminatedand pressure-bonded while being in an undried state.